1. Field of the Invention
The present invention relates to a flow adjusting valve and a die casting machine preferably incorporating the valve as a single unit, wherein the valve works to adjust the flow rate in a hydraulic circuit and to switch an injection operation from low speed to high speed.
2. Description of the Prior Art
In the die casting machine, melt is generally injected into the cavity of a metal mold from an injection cylinder at a low speed in the initial stage and at a high speed in the middle stage. The injection of the melt into the die casting machine is usually conducted by drive control of the injection cylinder, which is actuated by hydraulic pressure. To improve the quality of the molded products, low-speed injection must be switched to high-speed injection as quickly as possible and the injection speeds during the low-speed injection and the high-speed injection must be maintained as stably as possible.
In order to automatically adjust the flow rate to some degree, electromagnetic flow adjusting valves of the spool type have heretofore been employed. Electromagnetic flow adjusting valves control the flow rate in proportion to the input current and are equipped with differential transformers to move pilot spools in proportion to a current supplied to the differential transformers. Electromagnetic flow adjusting valves are further interlocked to main spools, so that the flow rate is maintained at a setpoint value. Electromagnetic flow adjusting valves are suitable for adjusting the flow rate to a desired value, but have no means to stop the main spool at a present position. Therefore, upon receipt of an external force, the main spools move back and forth in the axial direction with the present position as a center, whereby the flow rate undergoes variation and the injection speed of the die casting machine is not maintained constant.
In recent years, on the other hand, it has been required to arbitrarily set a pattern of injection speed to meet the metal mold in order to further improve the quality of the molded products. With the conventional flow adjusting valves of the spool type, however, the screw shaft is manually operated to set a flow rate. Therefore, it was impossible to arbitrarily set a pattern of injection speed. For this purpose also, electromagnetic flow adjusting valves of the spool type have also heretofore been used. In the electromagnetic flow adjusting valves of this type, however, the fact that the valve spools are actuated utilizing magnetic force makes it very difficult to precisely and quickly switch a small flow rate of, for example, 10 to 20 liters a minute to a large flow rate of, for example, 50 to 100 liters a minute with a maximum of 15,000 liters a minute, with just a single flow adjusting valve. Therefore, when electromagnetic flow adjusting valves of this type are adapted to a hydraulic circuit for driving the injection cylinder, a separate one must be provided for the small flow rate and for the large flow rate along with a switch valve.
Therefore, conventional electromagnetic flow adjusting valves of this type have not been able to satisfy such requirements as quick switching of injection speed, stable injection speed during the moment of switching, and simplicity of the hydraulic circuit. Accordingly, they have not been well suited for high-speed injection application, which requires stable injection speed and speed changes after short periods of time.
Further, even small amounts of foreign matter which is contained in the operation oil and which is adhered on the pilot spool, causes its movement to be changed and the flow rate to be changed.
Moreover, when the hydraulic circuit is switched, there develops shock in the hydraulic pressure due to the change of pilot lines and change of valves, to disturb operation of the main spool which has no means for mechanically anchoring it at the present position. Accordingly, it becomes difficult to accurately adjust the flow rate. This adversely affects the injection and the quality of the injection-molded products.
According to the conventional art, furthermore, a total of four valves were necessary; i.e., a switch valve for low-speed injection, a flow adjusting valve, a switch valve for high-speed injection, and a flow adjusting valve. Therefore, the hydraulic circuit tended to become complicated, requiring cumbersome control operation.
In connection with the above, it is noted that a flow adjusting valve of the spool type has heretofore been used in which one end of a cylinder chamber or a valve bore formed in the valve body serves as an inlet port for introducing the fluid that is to be controlled, and the opening degree of an outlet path formed in a side portion of the cylinder chamber is adjusted by moving the valve spool which is slidably provided in the cylinder chamber in the axial direction. However, in the conventional flow adjusting valve of this type in which the valve spool slides in direct contact with the inner surface of the valve body, the cylinder chamber must be formed in the valve body requiring a highly precise machining which is difficult to accomplish. Further, since it is difficult to absorb misalignment of axis between the cylinder chamber of the valve body and the valve spool, excessive force is exerted on the valve spool or on the driving portions, making it difficult to smoothly move the valve spool.